The present invention relates to calibration of a multichannel radar antenna and the associated software, and particularly to calibrating such an antenna and software in a missile in flight.
Missiles that use radar as part of their guidance systems generally have a radar antenna in the nose of the missile behind a radome. The radome includes a conical cap which is made of a radar-opaque material, typically metal. The balance of the radome forward of the radar antenna and behind the cap is made of a material transparent to radar.
The radar antenna is calibrated in the course of manufacture and initial setup. Typically, calibration is done in an anechoic chamber with a distant source of microwave radiation of known energy. This source is a far field source, meaning that its wavefronts are essentially parallel to the face of the antenna. The far field source of known energy provides a baseline for calibrating the radar antenna by adjusting variables in the associated software.
The radar antenna generally is arranged in a circular array divided (either physically or logically) into quadrants that meet at the center of the array. Each quadrant forms a separate channel in a multichannel radar antenna. The signals received by each channel of the antenna are transmitted to a processor for processing by software. To calibrate the antenna it is only necessary that a part of each channel of the antenna receive a far field burst of energy. Because the four channels of the antenna meet at the center, the antenna can be calibrated with a far field source that has a relatively small cross-section; covering only a section of each channel is sufficient.
Calibration of a radar antenna may be critical to its proper performance. This is especially true where sophisticated and sensitive software is used to interpret the received signals. For example, software used to distinguish the intended target from various decoys, jamming and/or camouflaging defensive measures associated with the target works better after calibration. Even if accurately calibrated during initial manufacture, the antenna""s response to incoming signals can vary over time. For example, after storage of the missile for a long period of time, the antenna can suffer slight physical changes which alter its response. In addition, the very act of launching a missile may subject it to forces and/or temperatures which alter its response.
Because the radar antenna""s response can change over time, there is a need for a system and apparatus that can be used to recalibrate a radar antenna in a missile while the missile is in flight.
The present invention provides a system and apparatus to recalibrate a multichannel radar antenna in a missile by simulating a far field source within the radome of the missile. A point source of radiation is located behind and inside the cap of the radome. Radiation from the point source (which produces spherical wavefronts) passes through a lens that causes the wavefronts to assume a parallel orientation. The parallel waves of radar energy hit the center area of the radar antenna, delivering a pulse of known energy to portions of each channel of the antenna. Based on this input, the software that processes the antenna""s signals is recalibrated to compensate for any change in antenna response from the original calibration.
The lens may be any conventional lens such as a lens with continuous concave and/or convex services, a Fresnel lens, a combination of such lenses, or even a diffraction grating. The lens may also utilize the inside surface of the radome as a reflective surface. Further, the lens could be replaced by a parabolic reflector or other device that simulates a lens.
The point source of energy may be a simple dipole antenna. The point source can be driven by an oscillator that can be powered in any of a variety of ways. Power can be fed through wires fastened to the inside of the nose cone or by a fiber-optic cable similarly secured. A laser can transmit energy through free space from the antenna to the oscillator, or the main radar transmitter can be used as an energy source with a capacitor or battery located in the metal cap of the radome to store the energy until it is required to power the oscillator.